Cycloolefin (co)polymers obtained by ring opening polymerization have features such as a high glass transition temperature due to the rigidity of the main chain structure, noncrystalline properties and high light transmittance due to the presence of a bulky group in the main chain structure, and low birefringence due to low anisotropy of the refraction and, by virtue of these features, have drawn attention as transparent thermoplastic resins possessing excellent heat resistance, transparency, and optical properties. Such cycloolefin (co)polymers obtained by ring opening polymerization are described, for example, in patent documents 1 to 6.
In recent years, the application of cycloolefin (co)polymers obtained by ring opening polymerization, for example, in the field of optical materials such as optical disks, optical lenses, and optical fibers, and sealing materials such as photosemiconductor sealing by taking advantage of the above features has been studied. Further, an attempt has also been made to apply the cycloolefin (co)polymer obtained by ring opening polymerization to films or sheets (films and sheets being hereinafter collectively referred to as “films”) for reducing the problems of the conventional optical films.
That is, films such as polycarbonate, polyester, or triacetyl acetate films, which have hitherto been used as optical films, have a problem of development of or a change in a phase difference upon a very small stress change due to the high coefficient of photoelasticity and a problem of heat resistance or hygroscopic deformation. In order to solve these problems, films comprising a cycloolefin (co)polymer obtained by ring opening polymerization have been proposed as various films for optical applications. For example, patent documents 7 to 10 describe retardation plates (an optical film having the function of imparting a phase difference to transmitted light; also known as a retardation film) comprising a film of a cycloolefin (co)polymer obtained by ring opening polymerization. Further, patent documents 11 to 13 describe the use of a film of a cycloolefin (co)polymer obtained by ring opening polymerization in protective films of polarizing plates. Further, patent document 14 describes a substrate for a liquid crystal display element formed of a film of a cycloolefin (co)polymer obtained by ring opening polymerization.
In recent years, an increase in size and an enhancement in function of liquid crystal display devices (LCDs) have led to a higher level of demands for retardation plates for use in LCDs. For example, for an increase in screen size and a reduction in weight of LCDs, evenness of the phase difference and freedom from blurring of optical axis have been demanded on a higher level. Further, for an improvement in viewing angle of LCDs, for example, the control of a phase difference in the thickness-wise direction has become required In order to meet these demands, homopolymers and copolymers obtained by ring opening polymerization of various cycloolefin monomers have been proposed as materials for retardation plates.
In the case of the homopolymers, however, the properties of the polymers obtained are disadvantageously determined unambiguously by the properties of the cycloolefin monomer used, and, hence, there is a limitation on meeting all the property requirements.
On the other hand, in the case of copolymers, stretching at a temperature around the glass transition temperature (hereinafter referred to also as “Tg”) of the copolymer sometimes causes serious problems such as cloudiness of the stretched film or a deterioration in uniformity of the phase difference. These problems can of course be avoided by carrying out stretching at an enhanced film stretching temperature. Stretching at an elevated temperature, however, lowers the developability of the phase difference and thus poses problems with the control of phase difference values, for example, an enhanced stretch ratio necessary for providing a desired phase difference value or the necessity of increasing the film thickness.
Accordingly, resins have been strongly desired which have excellent properties inherent in cycloolefin resins such as excellent heat resistance and transparency, do not pose any problem of cloudiness or the like upon film forming or film stretching at a relatively low temperature, for example, around Tg, and are suitable for applications of retardation plates and the like.
Under the above circumstances, the present inventors have made extensive and intensive studies and consequently have found that cycloolefin copolymers obtained by ring-opening polymerization, which have a specific structural unit, exhibit a single peak in a derivative differential scanning calorimetry curve obtained by differential scanning calorimeter (DSC), have a temperature width (hereinafter referred to as “Tg distribution”) of the rising part in the peak of 35° C. or below, and have a glass transition temperature of 110° C. or above, do not pose any problem of cloudiness or the like even upon stretching at a temperature around Tg and are suitable for applications of films or sheets and retardation plates. This has led to the completion of the present invention.    [Patent document 1] Japanese Patent Laid-Open No. 132625/1989    [Patent document 2] Japanese Patent Laid-Open No. 132626/1989    [Patent document 3] Japanese Patent Laid-Open No. 218726/1988    [Patent document 4] Japanese Patent Laid-Open No. 133413/1990    [Patent document 5] Japanese Patent Laid-Open No. 120816/1986    [Patent document 6] Japanese Patent Laid-Open No. 115912/1986    [Patent document 7] Japanese Patent Laid-Open No. 245202/1992    [Patent document 8] Japanese Patent Laid-Open No. 36120/1992    [Patent document 9] Japanese Patent Laid-Open No. 2108/1993    [Patent document 10] Japanese Patent Laid-Open No. 64865/1993    [Patent document 11] Japanese Patent Laid-Open No. 212828/1993    [Patent document 12] Japanese Patent Laid-Open No. 51117/1994    [Patent document 13] Japanese Patent Laid-Open No. 77608/1995    [Patent document 14] Japanese Patent Laid-Open No. 61026/1993